1. Field of the Invention
The present invention relates to an image recording apparatus for image recording on a recording material with plural recording elements, and more particularly to such image recording apparatus for dot recording with light emitting diodes as recording elements.
2. Description of the Prior Art
FIG. 1 schematically shows an example of a digital copier equipped with a recording unit for image recording with plural recording elements.
An original document placed on an original carriage is illuminated with an illuminating unit composed for example of a halogen lamp, and the reflected light is guided through mirrors 3, 4, and focused, by a lens 5, onto an image sensor 6 containing a linear array of plural photoreceptor elements. Said mirror 3 scans the original with a speed V in synchronization with the rotating speed of a photosensitive drum 7, and the mirror 4 is displaced with a speed V/2 to maintain a constant optical path length from the original to the lens 5. Consequently on the image sensor 6 there is formed, in successive manner, an image of a line of the pixels of the original image, thus generating image data of a line.
Along the periphery of the photosensitive drum 7 there are provided a high-voltage charger 8, a developing unit 9, a transfer charger 10, a cleaner 11 etc., and an LED array head 12 containing plural light-emitting diodes over the width of the photosensitive drum 7 is positioned perpendicular to the rotating direction thereof and at the downstream side of the high-voltage charger 8 with respect to the rotating direction indicated by an arrow. The light from the linear LED array is focused on the photosensitive drum 7 by an imaging unit 13.
The light-emitting diodes in said LED array 12 are turned on or off according to the image data of a line generated by said image sensor 6, and the light spots falling on the photosensitive drum generate an electrostatic latent image on said drum which is uniformly charged by the high-voltage charger 8. Said latent image is developed by the developing unit 9, and the developed image is transferred onto a sheet supplied between the transfer charger 10 and the photosensitive drum 7 and is fixed by heat or by pressure in a fixing unit 14 to obtain a copy of the original.
In such electrophotographic process, the dot corresponding to a lighted LED can be arbitrarily reproduced as black or white, by a suitable combination of the polarity of the toner and of the charging voltage. Stated differently, whether or not to deposit the toner on the drum illuminated by LED can be arbitrarily determined by the combination of the polarity of the charger and of the toner. The image sensor 6 is composed of a photoreceptor area containing an array of photoreceptor elements of a number corresponding to the number of pixels in a line for generating electric signals in response to the amounts of light falling thereon, and a signal shift area for serially releasing the electric signals received from said photoreceptor elements. In general such image sensor can be composed of a CCD image sensor mentioned before or a MOS image sensor. In the former, the charges corresponding to the amounts of light are supplied in parallel to a CCD shift register and are shifted by means of the shift function of the CCD for conversion into image voltage data. In the latter, the output signals from the photoreceptor elements are selected in succession by MOS switches and are converted into image voltage data.
FIG. 2 shows the electric connection between the image sensor 6 and the LED array head 12. In the image sensor 6, a linear array of m photoreceptor elements 21-1-21-m, composed for example of photodiodes, supplies electric signals corresponding to the received light in parallel manner to a shift register 22 to obtain, from a serial output terminal SO thereof, output signals corresponding to the received light which are converted into voltage signals by an amplifier 23. On the other hand, the LED array lead 12 has a linear array of m LED's 24-1-24-m, of which cathodes are grounded in common while anodes are respectively connected to the output terminals of drivers 25. The input terminals of said drivers are respectively connected to the output terminals of latches 26, of which input terminals are connected respectively to the parallel output terminals of a shift register 27. The serial output signals from the image sensor 6 are supplied to a binary encoding circuit 28 to obtain binary data for turning on or off the corresponding LED's in consideration of various conditions such as the magnitude of the received light, values of neighboring pixels, position of the pixel in consideration etc. The binary output signals are supplied to a shift input terminal S1 of the shift register 27 of the LED array head 12. A timing generator 29 determines the timing for parallel data entry from the photoreceptor element array 21 into the shift register 22, timing of data shift output from the shift register 22, timing of data input into the shift register 27 of the LED array head 12, and timing of simultaneous latching of data of a line in the shift register 27 by the latches 26 for turning on or off the LED's 24-1-24-m.
Consequently the LED 24-1 is turned on or off according to the intensity of light received by the photoreceptor element 21-1, and similarly the LED's 24-2-24-m are turned on or off respectively according to the light intensities received by the photoreceptor elements 21-2-21-m. ICK and OCK are clock signals, and ISG and OLG are gate timing signals of the shift registers 22, 27, respectively.
Thus, in response to a line image of the original formed on the image sensor 6, the LED's of the LED array head 12 are turned on or off according to the image density, thus forming an electrostatic latent image on the photosensitive drum 7 to enable copying of the original as explained before.
In the above-described system, the copied image is constituted by the dots irradiated by the LED's 24-1-24-m. Consequently the LED's 24-1-24-m should be made as small as possible in order to obtain a high image quality in the copied image. On the other hand, the LED's 24-1-24-m should be of a number at least covering the sheet width perpendicular to the direction of sheet feeding, for simultaneous recording of a line.
As a good image quality generally requires about 16 dots or pixels per millimeter, a copy of A3 size, in case of longitudinal feeding, requires 297 mm.times.16=4,752 LED's to be arranged at a density of 16 LED's per millimeter. Even in A4-size copying by longitudinal feeding, there are required about 4,000 LED's. Consequently, for the A3-size copying with longitudinal feeding mentioned above, there are required 4,752 independent drivers 25 and latches 26, and the shift register 27 has to be of a capacity of 4,752 bits. If these components are provided outside the LED head 12, the anode input terminals of the LED's have to be connected to the outside, but it is practically extremely difficult to extract more than 4,000 leads. It is therefore considered to form the shift register, latches, drivers etc. with integrated circuits and mount the same on the LED head 12.
However, though the use of integrated circuits allows mounting of said components on the head, a satisfactory high-speed operation cannot be achieved with a shift register as long as 4,752 bits. As an example, in case of producing about 40 copies per minute in A4 size with transversal feeding, each sheet has to be moved with a speed of about 270 mm/sec. In this case it is necessary to generate 270.times.16=4,320 lines of dots per second, so that the data have to be transferred at least at a rate of 4,320.times.4,752.apprxeq.2.times.10.sup.7 bits per second, or 20 Mbps. If the aforementioned integrated circuits are prepared for example with the I.sup.2 L process, the clock rate of the shift register is in the order of 1 Mbps. Consequently the conventional structure only allows to obtain several copies per minute, and is unable to exploit the advantage of high-speed electrophotographic copying process.
On the other hand, in case of placing dot-forming elements at a high density such as 16 elements per millimeter, the connections between said elements and driving elements have to be placed on one side of the dot-forming elements in case the parallel output signals from the shift register are connected with said dot forming elements through drivers as shown in FIG. 2, but it is extremely difficult in the practice to form connections with a density of 16 lines per millimeter.
Although the shift registers, latches, and drivers may be dividedly placed on both sides of the dot forming elements, it becomes necessary in such case to distribute the output signals from the binary encoding circuit to the shift registers of both sides bit by bit, and such distributing circuit cannot be formed on the printer head. There are therefore required complicated outside wirings.
Also, in case the image sensor 6 is composed of a CCD image sensor which is inevitably limited in size, the optical system has to be of a large dimension since the image of the original has to be focused on the CCD image sensor after size reduction through the optical system in order to read the image of a line. The optical system can be reduced in size, if a photoreceptor element array of a width same as that of the original is available, by focusing the image of the original on the photoreceptor element array in actual size for example through a rod lens array. However, it is not possible to linearly align plural CCD linear image sensors by cutting off the end portions of such sensors and leaving only the effective areas thereof. It is therefore proposed to provide a linear image sensor capable of reading the image of a line in actual size by arranging plural CCD linear image sensors in a staggered pattern, though a line on the original cannot be read at the same time.
In such case, since neighboring CCD linear image sensors are aberrated by plural dot lines on the original, there is required a memory for storing image data of said plural lines in order to reproduce the original with the conventional dot line printer head equipped with dot forming elements of a line. There is thus required a complicated expensive control circuit for the dot line printer head.
On the other hand, there is already known, as shown in FIG. 3, an LED printer head containing a linear array of a plurality of LED's. In such LED printer head, said linearly arranged plural LED's are divided into plural blocks each containing a determined number of LED's. For example, in order to record 4,096 dots, there can be employed an LED printer head composed of 64 LED chips each constituting a block of 64 LED's. These LED's are subjected to dynamic drive by switching in the unit of blocks in successive manner by means of switching elements such as transistors. In FIG. 3 there is shown an LED substrate 102 on which mounted in a multi-layered printed circuit board 103 having matrix wirings composed of multiple layers of epoxy resins or ceramic materials, and on said circuit board 103 mounted are LED chips LC1-LC64. A common electrode of each of the LED chips LC1-LC64, each containing a determined number of LED's, is positioned on the bottom face of the chip and is connected with one of common leads CB1-CB64 provided on the multi-layered circuit board 103. Segment electrodes of the 4096 LED's L1-1, L1-2, . . . , L64-64 in the LED chips LC1-LC64 are bonded with wires to segment matrix wirings 104a, 104b.
FIG. 4 is a partial magnified view of the bondings between said segment matrix wirings 104a, 104b and the LED chips LC1-LC64. The anodes of the LED's L2-1, L2-2 of the LED chip LC2 are connected within the chip with wirings CW2-1, CW2-2, which are connected through bonding wires BW2-1, BW2-2 with segment lead bands SB2-1, SB2-2. In insulating layers 105a, 105b, said segment lead bands SB2-1, SB2-2 are respectively connected with segment bus lines SBL1, SBL2 through holes H2-1, H2-2. Though the foregoing explanation is limited to LED's L2-1, L2-2 in the LED chip LC2, it is to be understood that similar connections are made to other LED's L1-1-L64-64. The segment bus lines SBL1-SBL64 are connected to a printer head driving circuit through cables 106a, 106b composed for example of flexible printed circuit boards as shown in FIG. 3.
FIG. 5 shows a known circuit for driving the LED printer head described above. In FIG. 5 there are shown an LED printer head 101; a video signal input terminal T1 for entering serial pixel data signals 107; a terminal T2 for entering clock signals 108; a terminal T3 for entering a video enable signal; a 64-bit shift register 110 for storing the pixel data signals 107 in synchronization with the clock signals 108; a 64-bit latch circuit 111; a 64ry counter 112 for counting the clock signals 108; a delay circuit 113; a 64ry counter 114; a decoder 115; and switching elements SA1-SA64, SB1-SB64 for driving the LED printer head 101 in succession. The LED printer head 101 contains 4096 LED's which are divided into 64 blocks each containing 64 LED's.
In the following there will be given an abbreviated explanation on the function of the LED printer utilizing the above-described driving circuit.
(1) First 64 bits of the serial pixel data signals 107 are entered into the shift register 110, and are stored in the latch circuit 111 in response to a count-up signal 116 of the 64ry counter 112 to be released later.
(2) The switching elements SB1-SB64 are respectively turned on or off according to the output signal of the latch circuit 111. Subsequently a latch clear signal 117 released from the delay circuit 113 clears the latch circuit 111, whereby all the switching elements SB1-SB64 return to the off state.
(3) The output signal of the 64ry counter 114 counting the number of latch clear signals 117 is supplied to the decoder 115, thereby turning on one of the switching elements SA1-SA64 for driving a determined LED block or chip. The switching elements SA1-SA64 and SB1-SB64 are controlled in synchronization with the clock signals 108 and the video enable signal 109.
(4) The foregoing steps (1) to (3) are repeated 64 times to reproduce 4096 (=64.times.64) pixels by the LED printer head 101. In this manner the dynamic drive corresponding to a scanning line is completed.
In forming a latent image corresponding to the original image by irradiating the photosensitive drum with the above-described LED printer head 101, the light emission from the LED's has to be increased in order to increase the image forming speed. Though the light emission can be increased to a certain extent by increasing the driving current for the LED's, an LED chip will require a current of 3.2 A in case all the LED's are turned on with a driving current of 50 mA per LED. In such case the current in each of the segment bus lines SBL1-SBL64 is 50 mA at maximum, but a pulse current of 3.2 A with a duty ratio of 1:64 flows in each of the wirings W1-W64 leading from the common electrodes of the LED chips LC1-LC64 to the switching elements SA1-SA64 through common lead bands CB1-CB64. 4096 LED's provide an image width of 256 mm with a pixel density of 16 pixels/mm. However, in order to achieve an image forming speed of 200 mm/sec. corresponding to about 30 sheets/min. in A4 size in the Japan Industrial Standards, the switching elements SA1-SA64 are required to switch a current of 3.2 A with a pulse duration of ca. 4.9 .mu.sec. Because of the inductances present in the connections between the switching elements SA1-SA64 on the driver board and the common lead bands CB1-CB64 on the LED printer head 101, the current in each LED chip shows a slight lag after the switching element is turned on. Consequently the light emissions from the LED's L1-1-L64-64 become lower even if the peak current in each LED is equal to 50 mA. However, in case only one LED is lighted in each LED chip, such loss in the light emission is not observed since the current in the common electrode is limited to 50 mA, thus ensuring a rectangular wave form. In this manner the light emission per LED varies according to the number of lighted LED's in each chip. In addition the resistances in the wirings W1-W64 leading from the common lead bands CB1-CB64, there are generated voltage drops between the common electrodes the LED chips and the switching elements according to the current flowing therebetween. In order to reduce the influence of such inductances and impedances there should be employed thick wirings, but it is practically difficult to employ 64 thick wirings as a set. Consequently such LED printer head can only be employed in a low-speed LED printer with a low light emission and with a small total current per LED chip.
On the other hand, in order to obtain a print with A3 size with a pixel density of 16 pixels/mm, there is required an array of ca. 4,800 LED's for covering the shorter side of ca. 300 mm of the A3 size. More specifically, a total length of 296 mm can be covered with 4,736 LED's divided for example into 37 LED blocks each containing 128 LED's. In case of matrix drive of such 37 LED blocks, the lighting time of an LED block is only 1/37 of the time required for printing pixels of a line. Also in case of using 64 LED blocks each containing 64 LED's as explained above, the lighting time of an LED block is only 1/64.
Consequently, in comparison with a case of connecting drivers and latches in parallel manner to 4,736 LED's and releasing image signals of a line in parallel manner from a serial-to-parallel converter of 4,736 bits to turn on or off the LED's in response to said image signals over the entire pixel printing period for the line, the LED's are required to a 37-times stronger intensity for a same printing speed.
The LED array chip employed in the conventional LED printer head is principally made of GaAsP, and has an output power of ca. 8 .mu.W with a driving current of 10 mA per LED, for a pixel density of 16 pixels/mm and at a wavelength of 650 nm. On the other hand the converging fiber array for focusing the light onto the photosensitive drum only has an efficiency of ca. 10% at maximum. Consequently an energy of ca. 1 .mu.J/cm.sup.2 is required even with a photosensitive drum having a high sensitivity in the region of 650 nm. In this case the image formation be relative standstill relation of the photosensitive drum and the LED array chip requires about 49 .mu.sec. In case of the matrix drive of the LED array chips mentioned above, the printing time for a line is increased by 37 times and becomes equal to 1.8 msec. This time, being required for printing 1/16 mm, corresponds to a printing speed of ca. 34.5 mm/sec. or ca. 12 seconds per A3-sized copy. In order to reduce this printing time to the order of 4 seconds, the light emission has to be increased 3 times by a corresponding increase in the driving current for LED's. However, such increase in the driving current curtails the service life of the LED. For example, in order to ensure, after a use of 100 hours, a light intensity of at least 60% of the initial intensity, the current density in the LED should not exceed 200-300 A/cm.sup.2 for a maximum junction temperature of 80.degree. C. The area of an LED for achieving a density of 16 pixels/mm is 62.5.times.62.5 .mu.m, so that a current of 10 mA induces a current density of 356 A/cm.sup.2. Thus a current exceeding 10 mA is undesirable also in consideration of the service life of LED.
If each LED is driven with a current of ca. 30 mA without the consideration on the service life, the current required in the common electrode for lighting all the LED's in each block amounts to 3.84 A. Also the active period for each block is ca. 16 .mu.sec, for printing A3 size in 4 seconds. However the high-speed switching of a current as large as 3.84 A inevitably involves a delay of several microseconds. Said delay time depends on the magnitude of current, so that the light intensity from each LED fluctuates according to the number of lighted LED's in each block. The conventional printer head cannot therefore provide a high image quality, and can only be utilized in low-speed printers.